The electric motors drive shaft-mounted propeller fans for moving air through air-cooled condenser coils, in air conditioning and refrigeration packages, for instance, operate as many as 10 fans on the package with the fans applied in step multiples. The fans are operated in steps to control the refrigerant pressure in the condenser. It is common for some fans to be operating while others are idle. Backflow of air through idle fans often causes them to "windmill" in the reverse direction. Ambient winds can also cause windmill of idle fans.
If the electric motors are single-phase permanent split capacitor motors, they will startup in the wrong direction, if they are already rotating in the wrong direction when power is applied. This causes the power fan attached to the motor to move air in the wrong direction and in the environment of use in air conditioning and refrigeration packages, by so doing, causes excessive condensing pressure in the unit, resulting in shutdown upon safety high pressure cutout.
If the electric motors are three-phase motors, they will not startup in the wrong direction, if rotating in the wrong direction when power is applied. However, when power is applied, they will come to a stop very quickly and immediately proceed to accelerate in the correct direction. This rapid transition from backward or wrong to forward or right direction produces an excessive torque load at the prop fan attachment point and fans have been known to shear hubs and blades due to this torque. Such a fan blade, in all likelihood, would be destroyed if the three-phase motor was started while it was spinning in the wrong direction.
Fan back-stops have been proposed using a roller clutch, hardened steel sleeve pressed onto the motor shaft and an outer sleeve prevented from rotating by a radius rod tied to the motor frame to prevent its rotation. While such devices may have utility, the devices involve relatively rotating parts in contact with each other, constituting a bearing device requiring lubrication, and are fairly complicated and relatively expensive.
It is, therefore, an object of the present invention to provide an electric motor back-stopping device which effectively prevents backward rotation of an electric motor caused by torque load reversal on the motor shaft during any time the motor is turned off, which device is essentially frictionless, is free of relative motion between parts during continuous motor ON or OFF cycle, needs no lubrication, is not affected by atmospheric particulates, imposes no measurable load on the motor shaft, works in any position, and is quite inexpensive.